The purpose of the proposed study is to use neuropsychological test and diffusion tensor imaging (DTI) to investigate the impact of white matter abnormalities on cognitive functioning among non-drug and alcohol abusing patients infected with Human Immunodeficiency Virus (HIV). Recent studies demonstrate the sensitivity of DTI in detecting white matter changes associated with HIV infection when compared to traditional structural MRI sequences. Preliminary data from our laboratory suggest that changes in the diffusion characteristics of HIV infected patients is also a better predictor of neuropsychological performance. The resent study will extend these preliminary data and examine in more detail the relationships between disease severity, neuropsychological testing and neuroimaging variables. To accomplish this, we will recruit two experimental HIV groups including 30 HIV infected non-drug/alcohol abusing patients with CD4 cell counts below 350 and 30 HIV infected non-drug/alcohol abusing patients with CD4 cell counts greater than350 and compare them to 20 demographically matched controls. Each participant will undergo neuropsychological testing and DTI neuroimaging. Differences between each of the groups will be investigated with the expectation that DTI variables will be more significantly altered in patients with thelowest CD4 cell counts. We also expect that the DTI variables for the control participants will be significantly different from the HIV patients with a higher CD4 cell count. We also expect that the DTI variables will be significantly predict changes in neurocognitive functioning among HIV infected patients and that DTI variables will be significantly related to disease severity. Though DTI has been used to investigate the neuropathological changes in HIV infected patients, this study will be one of the first to investigate these changes and their impact on cognitive functioning across multiple domains relative to measures of disease burden.